Conventionally, as monomers which provide polymers used for ophthalmic lenses, compounds having a silicon group have been known.

As one of such compounds, those shown in the above-mentioned formula (c) or (c′) are known (for example, patent reference 1). This compound has a feature to be easily compatible with a hydrophilic monomer since it has a hydroxyl group in its molecule, and there is a well-known synthetic method thereof (patent reference 1). However, by the method described in the patent reference 1, even if various reaction conditions are tested, purity can be increased to only from 85% to about 89%, but a higher purity has been desired.
Patent reference 1: JP-A-S56-22325, Example 4